The hydraulic bulging has many features compared with other forming methods. For example, a component having a complicated shape in which a sectional shape is varied in a longitudinal direction can be produced by the hydraulic bulging, so that a mechanical component in which welding is required in the conventional technique can be produced by integral forming. In the hydraulic bulging, work hardening is generated in the whole of the produced component, so that high-strength product can be obtained even if the mild mother pipe is used.
Furthermore, little springback is generated after the hydraulic bulging, and good dimensional accuracy is obtained in the product (good shape fixability). Therefore, a process of adjusting the product dimension is not required, and the process can be streamlined.
Recently, the above excellent features of the hydraulic bulging are highly regarded and the hydraulic bulging is particularly being adopted as a method of producing automobile components.
FIG. 1 is a view explaining the conventional hydraulic bulging in which a straight pipe is used, FIG. 1A shows a pre-hydraulic bulging sectional configuration, and FIG. 1B shows a post-hydraulic bulging sectional configuration.
In the hydraulic bulging in which a general straight pipe is used, as shown in FIG. 1A, a working fluid is injected through an injection hole 3 into a mother pipe P1 set in a pair of upper and lower dies 1 and 2, and pressing working is performed from both pipe ends toward an axial direction by axial pressing tools (hereinafter referred to as “sealing tool”) 4 and 5 which are also used as a sealing tool, while working fluid pressure (hereinafter referred to as “inner pressure”) is increased. This enables a product P2 having a sectional shape shown in FIG. 1B to be produced.
During the hydraulic bulging, the sealing tools 4 and 5 are connected to a hydraulic cylinder (not shown) to control a position in an axial direction or axial pressing force.
In the hydraulic bulging, it is said that axial pressing working in the axial direction to be applied from a pipe end is an extremely important working step, because the axial pressing working promotes a metal flow during expansion to improve an expansion limit.
That is, in the hydraulic bulging, a pipe wall thickness is remarkably decreased according to the expansion of the material, only when the internal pressure is simply applied while the positions in the axial direction of both end portions of the mother pipe is fixed without performing the axial pressing working from the pipe end. Therefore, fracture is generated in the midway of the hydraulic bulging, and a formable range (expansion limit) is restricted.
In the hydraulic bulging, there is another problem caused by a shape of the mother pipe. As described above, one of the features of the hydraulic bulging is that the complicated shape in which the sectional shape is varied in the axial direction can be produced by the hydraulic bulging. However, there is a limitation in the work shape obtained by the conventional hydraulic bulging.
For example, a peripheral length increasing rate (expansion ratio is defined as peripheral length increasing rate (expansion ratio)={(outer peripheral length of the instant portion of the workpiece/circumferential length of mother pipe)−1}×100%, depending on shape characteristics necessary for the working product and mother pipe conditions (material grade and pipe wall thickness) to be used, the peripheral length increasing rate (expansion ratio) is about 25% at most except for the pipe end portion area where the axial pressing is effective.
That is, further elaboration is required in order to increase a degree of freedom of the product shape design and to obtain the product having the further complicated, arbitrary sectional shape.
In order to solve the problem, instead of the straight mother pipe, there is proposed the use of a substantially conical mother pipe (hereinafter referred to as “tapered mother pipe”) having a peripheral length in which an outer diameter is gradually increased or decreased from one end to the other end in the axial direction.
Specifically, when the tapered mother pipe is used, the peripheral length increasing rate associated with the hydraulic bulging can be suppressed to a lower level to form a predetermined work shape, even in the component in which the forming is hardly performed using the straight mother pipe, e.g., the component in which the peripheral length is largely varied along the axial direction (for example, see FIG. 2 in page 1 of Japanese Patent Application Publication No. 2001-321842).
However, in the case where the hydraulic bulging is performed with the tapered mother pipe in which the sectional shape is varied in the axial direction, it is difficult that the axial pressing is performed to the tapered mother pipe using the sealing tool for the straight mother pipe shown in FIG. 1.
FIG. 2 is a view explaining a problem generated in the case where the axial pressing is performed to the tapered mother pipe with a conventional straight mother pipe axial pressing tool. As shown in FIG. 2, on the large-diameter end side, the axial pressing cannot be performed to the tapered mother pipe TP1. On the small-diameter end side, the axial pressing can be performed to the tapered mother pipe TP1. However, as the axial pressing tool 4 intrudes in the upper and lower dies 1, 2 in association with the axial pressing, constraint of inner and outer surfaces of the tapered mother pipe TP1 becomes insufficient on the side of the axial pressing tool 4, which results in generation of sealing leakage.
FIG. 3 is a view explaining a hydraulic bulging process with the conventional tapered mother pipe, FIG. 3A shows a sectional configuration in which the tapered mother pipe which is of the starting material is set prior to the forming, FIG. 3B shows a pre-hydraulic bulging sectional configuration in which a profile mother pipe for hydraulic bulging is formed, and FIG. 3C shows a sectional configuration when the hydraulic bulging finishes.
In the conventional hydraulic bulging in which the tapered mother pipe TP1 is used, sealing tools 6 and 7 whose front-end portions are tapered are used as shown in FIG. 3. However, usually the hydraulic bulging is completed only with the internal pressure load because the axial pressing cannot be performed. In FIG. 3, the symbol TP2 designates a tapered mother pipe after the pipe end portion is formed, and the symbol TP3 designates a product (hydraulic bulged product) after the hydraulic bulging.
In the hydraulic bulging process shown in FIG. 3, because the axial pressing cannot be performed to the tapered mother pipe TP2, as described above, the working can be performed within a limited forming range where the fracture is not generated in the hydraulic bulging stage. Accordingly, in the hydraulic bulging, currently the effect generated by the use of the tapered mother pipe is not sufficiently exerted.
Therefore, in the case where the hydraulic bulging is performed using the tapered mother pipe, there is demanded a technological development in which the axial pressing can be performed from the pipe end toward the axial direction in addition to the internal pressure load on the mother pipe.